Calcium bentonite clays, i.e., clay in which the principal exchangeable cation is a calcium ion, are also referred to as sub-bentonites, calcium montomorillonites or calcium smectites. Generally, these are hydrated aluminosilicate crystalline minerals. Usually, magnesium proxies for some of the aluminum in the crystals of the clay. Iron content varies with clays from different deposits.
For many years, selected bentonite source clays have been treated on a commercial scale with acids to leach aluminum from the structure. The acid leaching has been practiced to produce bleaching earths, cracking catalysts and reactive pigments for carbonless copying paper, among other commercial applications. See for example, patents and publications cited in U.S. Pat. Nos. 5,008,226 and 5,008,227.
The starting clays which are used to produce known forms of acid leached bentonites typically contain approximately 20% alumina (based on the dry weight). The aluminum in bentonites are in octahedral and tetrahedral bonding structures. Acid dosages of about 40-50 gm of 96% H.sub.2 SO.sub.4 /100 gm clay are typically used. Alkaline earth and alkali metals are removed. The clays are usually leached to a residual aluminum content in the range of about 10-15 wt. %. The extent of leaching varies inter alia with the intended use of the leached clay. However, in general practice, both octahedral and tetrahedral aluminum remain in the solid residue which, when studied by XRD, exhibits lines characteristic of the clay crystals. The acid treated clay is invariably washed to remove soluble salts and entrained acid. While sulfuric acid is usually the acid of choice, other acids such as phosphoric and citric acids have been proposed.
It is known that repeated sulfuric acid leaches, resulting in extractions in excess of those used in the typical commercial prior art practice, can produce siliceous residues with essentially no aluminum. The porosity (surface area and pore volume) can be severely destroyed by such practice. This may explain why exhaustive leaching to remove virtually all aluminum has not been practiced commercially.
Acid-activated bentonites have been used as reactive pigments for several decades for paper products, in particular for use as a porous pigment in carbonless copy paper manufacture. The acid-leached bentonite was used with normally colorless leuco dyes to develop colored images. In the case of U.S. Pat. No. 4,405,371, Sugahara et al., proposed to use a relatively highly leached bentonite. The bentonite was leached by H.sub.2 SO.sub.4 or HCl to such degree that SiO.sub.2 content was about 82-96.5 wt %, preferably 85-95 wt %. The acid-leached bentonite was characterized by its loss of X-ray crystallinity, regardless of its aluminum content or structure. However, it was noted that the acid-leached bentonite had a relatively low BET surface area, about 180 m.sup.2 /g.
The use of acid-activated bentonite as petroleum cracking catalysts was proposed in the 1930s. The mild acid-leaching, usually using H.sub.2 SO.sub.4 or HCl, generated porosity and acidity which were required for the catalysis. After the acid-leaching, part of the clay crystallinity was maintained, and most aluminum still remained in the structure, about 10-15 wt. % expressed as Al.sub.2 O.sub.3. It was found that the catalytic activity was reduced after more aluminum was removed.
The following references are nonlimiting examples of references that relate to the preparation of acid-leached bentonites and to their use in catalytic cracking applications:
R. E. Grim, "Applied Clay Mineralogy", McGraw-Hill, New York, 1962, p. 307-332. PA1 G. R. Bond, "Acid-treated clay catalyst for cracking hydrocarbons", U.S. Pat. No. 2,551,580 (1951). PA1 A. Grenall, "Montomorillonite cracking catalyst, X-ray diffraction", Ind. Eng. Chem., 40 (1948) 2148-2151. PA1 U.S. Pat. No. 3,944,482, Bruce R. Mitchell et al., "Process for the Cracking of High Metals Content Feedstocks".
Since the discovery of zeolites in the 1960s, the role of acid-activated bentonite in refinery cracking has substantially decreased except for some uses such as a matrix constituent. For example, in Mitchell et al patent (U.S. Pat. No. 3,944,482) acid-leached bentonite was used as matrix for high metals tolerant resid catalysts. Mitchell et al found that the acid-activated bentonite matrix must meet two requirements: (1) it must have a high aluminum content above 20 wt %, and (2) its average pore diameter must be larger than 100 A.